Ghost images, commonly referred to as "ghosts", are a common occurrence in received television pictures. The signal to which the television receiver synchronizes is called the reference signal, and the reference signal is usually the direct signal received over the shortest transmission path. The multipath signals received over other paths are thus usually delayed with respect to the reference signal and appear as trailing ghost images. It is possible however, that the direct or shortest path signal is not the signal to which the receiver synchronizes. Where the receiver synchronizes to a reflected (longer path) signal, there will be a leading ghost image caused by the direct signal, or there will a plurality of leading ghosts caused by the direct signal and other reflected signals of lesser delay than the reflected signal to which the receiver synchronizes. The multipath signals vary in number, amplitude and delay time from location to location and from channel to channel at a given location. The parameters of a ghost signal may also be time-varying.
The visual effects of multipath distortion can be broadly classified in two categories: multiple images and distortion of the frequency response characteristic of the channel. Both effects occur due to the time and amplitude variations among the multipath signals arriving at the reception site. When the relative delays of the multipath signals with respect to the reference signal are sufficiently large, the visual effect is observed as multiple copies of the same image on the television display displaced horizontally from each other. These copies are sometimes referred to as "macroghosts" to distinguish them from "microghosts", which will be presently described. Usually, the direct signal predominates, and a receiver is synchronized to the direct signal. In such case the ghost images are displaced to the right at varying position, intensity and polarity. These are known as trailing ghosts or "post-ghost" images. In the less frequently encountered case where the receiver synchronizes to a reflected signal, there will be one or more ghost images displaced to the left of the reference image. These are known as leading ghosts or "pre-ghost" images.
Multipath signals of relatively short delay with respect to the reference signal do not cause separately discernible copies of the predominant image, but introduce distortion into the frequency response characteristic of the channel. The visual effect in this case is observed as increased or decreased sharpness of the image and in some cases loss of some image information. These short-delay or close-in ghosts are most commonly caused by unterminated or incorrectly terminated radio frequency transmission lines such as antenna lead-ins or cable television drop cables. In a cable television environment, it is possible to have multiple close-in ghosts caused by multiple taps with improperly terminated drop cables of varying lengths. Such multiple close-in ghosts are frequently referred to as "micro-ghosts".
in the prior art, long multipath effects, or macroghosts, are typically reduced by cancelation schemes. In the prior art short multipath effects, or microghosts, are typically alleviated by waveform equalization, generally by peaking and/or group-delay compensation of the high frequency video response.
The phenomenon of television ghosts has been addressed with a view to improving picture quality by reducing or eliminating ghosts. See, for example, W. Ciciora et al., "A TUTORIAL ON GHOST CANCELING IN TELEVISION RECEIVERS", IEEE Transactions on consumer Electronic, vol. CE-25, February 1979, pp. 9-43. Other solutions to the problem of ghosts are described in U.S. Pat. No. 4,896,213, Jan. 23, 1990, Kobo et al. and U.S. Pat. No. 4,897,725, Jan. 30, 1990, Tanaka et al., the disclosure of which is herein incorporated by reference.
Since the characteristics of a transmitted television signal are known a priori, it is possible, at least in theory, to utilize such characteristics in a system of ghost signal detection and cancelation. Nevertheless, various problems limit this approach. Instead, it has been found desirable to transmit repeatedly a reference signal situated, for example, in a section of the TV signal that is currently unused for video purposes and to utilize this reference signal for detection and cancelation of ghost signals. Typically, lines in the vertical blanking interval (VBI) are utilized. Such a signal is herein referred to as a Ghost Canceling Reference (GCR) signal.
The strategy for eliminating ghosts in a television receiver relies on the transmitted GCR signal suffering the same multipath distortions as the rest of the television signal. The receiver can then examine the distorted GCR signal it receives and, with a priori knowledge of the waveform of a distortion-free GCR signal, can configure an adaptive filter to cancel, or at least significantly attenuate, the multipath distortion. It is important to choose a GCR signal that does not take up too much time in the VBI (preferably no more than one TV line), but that still contains sufficient information to permit the receiver to analyze the multipath distortion and configure an compensating filter to cancel the distortion.
It has been proposed that a useful test or GCR signal may appropriately exhibit a (sin x)/x waveform. Such a waveform, suitably windowed, exhibits a relatively constant spectral energy density over a frequency band of interest. See, for example, the above-mentioned tutorial paper by W. Ciciora et al. Ghost locations can then be determined for ghost signal cancelation and waveform equalization to reduce the affects of short multipaths.
The above-mentioned U.S. Pat. No. 4,896,213 discloses a ghost canceling signal transmission/reception system which allows a built-in ghost canceling device to reduce or eliminate ghost components attributable to group-delay distortion and frequency-amplitude characteristic distortion generated in a signal transmission path. This is achieved by superimposing a digital signal on a television signal as a ghost canceling reference signal. Thus, as disclosed in the above-mentioned U.S. Pat. No. 4,896,213, a digital signal composed of frame synchronizing signals, clock synchronizing signals, and data signals is generated. This digital signal is superposed, during the vertical blanking interval, on a television signal to be transmitted. At the receiving end, the digital signal superposed on the television signal is utilized as a reference signal in an arrangement that executes a correlative operation of the transmitted television signal to compute the parameters for the adaptive filter circuits used to reduce the ghost phenomena.
In the arrangement disclosed in the above-mentioned U.S. Pat. No. 4,897,725, a transmitted reference or GCR signal is also used. A dummy ghost signal is generated and is utilized for canceling a ghost signal in the transmitted television signal. This is substantially the proposed BTA (Japan) GCR signal, which utilizes as the main reference or deghosting signal, a signal having aformentioned (sin x)/x waveform, principally for its property of spectral energy content uniformly distributed throughout the frequency domain. Averaging with a pair-wise constant signal is utilized for deriving a received reference waveform. The received reference waveform is Fourier transformed to provide a set of Fourier coefficients. The transformed reference waveform is then processed with an available FFT of an unimpaired GCR to compute the deghosting filter parameters, that is, tap gain information for a transversal filer, for both waveform equalization (finite impulse response, FIR) and the deghosting filter (infinite impulse response, IIR).
As can be expected, the GCR signal is generally received accompanied by its ghost signals and is thus itself a "ghosted" signal. As evidenced in U.S. patent application Ser. No. 07/609,522 filed 5 Nov. 1990, the inventor recognized that the performance of a ghost canceling system is greatly influenced by the noise and perturbation content of the acquired GCR signal. The inventor also recognized, as evidenced in his patent application Ser. No. 07/609,522, that a reduction in the noise and perturbation content of the acquired GCR signal is desirable in improving the accuracy of the deghosting filter parameter derivations and in reducing the system complexity.
The inventor further recognized, as evidenced in patent application Ser. No. 07/609,522, that a step in the signal leading edge is desirable in a GCR signal in computing ghost locations. As previously mentioned, a (sin x)/x waveform provides particular advantages in a GCR signal: its flat frequency spectrum allows accurate computation of the filter parameters for attenuating multiple image effects as well as computation of the waveform equalizing parameters. However, the characteristic ripples of the (sin x)/x waveform, along with other high frequency components, are typically attenuated in a received ghosted GCR, both due to multipath effects as well as effects of antenna misorientation as commonly occurs in practice. Under such conditions, the computation of the waveform equalizing parameters can be significantly in error. These problems are particularly apparent when a (sin x)/x step is utilized for the leading edge, as in the proposed BTA (Japan) GCR signal.
The initial portion of the GCR signal, then, should be substantially ripple-free. Too rapidly rising an initial step will contain substantial high-frequency content that is outside the bandwidth constraint imposed by the broadcast television standard. The filtering of the video signal at the transmitter to keep the video signal within that bandwidth constraint will remove the above-band frequency content, giving rise to ringing in the step edge. This ringing is Gibbs's phenomenon. A 2T type step has a 10 to 90% rise-time of 250 nanoseconds and a frequency spectrum to 4 MHz; in a television transmission system having 6 MHz video signal bandwidth this step is transmitted substantially without attendant ringing.
A simple step does not contain all frequencies, however, and a 2T type step does not provide as much high frequency energy as desired. These deficiencies hamper the computation of weighting coefficients for filtering to equalize across band the amplitude response from the ghost cancellation circuitry. Using an FIR filter for the equalization of the amplitude response across band results in a concomitant linearization of phase response and suppresses microghosts.
When the high frequency energy content of the initial portion of the GCR signal is kept low to avoid ripple therein, the final portion of the GCR signal should include the high frequency energy content required for computing weighting coefficients for equalization across band. The inventor suggested in U.S. patent application Ser. No. 07/609,522 that a GCR signal including a pulse with a leading edge 2T type step, a trailing edge (sin x)/x step and a sustained relatively constant level therebetween met his criteria for a good GCR signal.
A GCR signal using Bessel chirps subsequently publicly proposed by U.S. Philips Corp. also meets the inventor's criteria for a good GCR signal. The distribution of energy in the Bessel chirp signal has a frequency spectrum extending continuously across the composite video signal band. The chirp starts at the lowest frequency (about 200 kHz) and sweeps upward in frequency therefrom to the highest frequency (about 4.2 MHz). The chirp is inserted into the first halves of selected VBI lines. Bessel pulse chirps, each of 35 microsecond duration, begin 12 microseconds into the 19.sup.th VBI scan lines of each cycle of eight successive fields with chirp polarity alternating from field to field within each frame and being reversed each second frame. These chirps swing plus/minus 40 IRE from 30 IRE "gray" pedestals which extend from 12 to 48 microseconds into these VBLI lines. Because of the similarities of the Bessel pulse chirp signal to the GCR signal that includes a leading edge 2T type step, a trailing edge (sin x)/x step and a relatively constant level therebetween, the signals can be processed similarly to effect ghost cancelation. Both signals are contained within an envelope of about 80 IRE units.